Conventionally, protective relay systems are utilized to protect power system facilities, such as a power transmission line, a bus bar, and a transformer constructing a power system, by detecting and eliminating an accident that occurs in the power system facilities. For example, as to a protective relay system that protects a power transmission line, there is a technology which provides a protective relay device at a section end of the power system, connects respective protective relay devices via transmission channels, and detects an accident based on electrical quantity information at each terminal.
Among such protective relay devices, a current differential relay device that protects a power transmission line obtains current value information from the local terminal, and transmits the current value information to the terminal of a communication-partner device via a transmission channel. Next, the communication-partner protective relay device that has received the current value information through the terminal detects an accident of the power transmission line based on a difference between current value information obtained from the local terminal and the current value information received via the transmission channel. A time (sampling time) at each terminal of the current difference relay device needs to be synchronized highly precisely.
For a transmission between the protective relay devices in conventional protective relay systems, conventionally, microwaves of substantially 54 Kbps to 1.5 Mbps or optical lines are applied, and a highly precise synchronization is realized by transmitting synchronization frames with each other. In recent years, application of a general-purpose network like Ethernet (registered trademark) which is faster than microwaves and optical lines, and which is advantageous in maintenance and costs is being examined. According to a protective relay system utilizing a general-purpose network, a communication is performed among plural protective relay devices via communication relaying devices. Hence, the process timing of a synchronization frame in the communication relaying device may overlap the process timings of other frames, and the forwarding time of the synchronization frame may be delayed.
When the forwarding time of the synchronization frame is delayed, a difference is caused in the outward transmission time between the protective relay devices and the return transmission time between the devices, and thus the synchronization precision decreases. When the synchronization precision decreases, the current differential relay device performs a differential computation at a different sampling timing, and thus a false operation may occur.
[Related Technical Documents]
[Patent Documents]
[Patent Document 1] JP 2010-56947 A